1. Field of the Invention
The present invention relates to a manufacturing method for molded products comprising a heat-curable resin (thermo-setting resin) such as unsaturated polyester resin, epoxy resin, urethane resin, diallylphthalate resin, or the like, which is capable of preventing the occurrence of voids or concavities (concave portions) on the surface of the molded products caused by the shrinkage in volume accompanying the curing of the heat-curable resin.
2. Description of the Related Art
This type of heat-curable resin experiences a comparatively large volume shrinkage during curing, so that for example, unsaturated polyester resin shrinks by as much as 7%. For this reason, when molded products are molded using a injection method, xe2x80x9cconcavitiesxe2x80x9d are generated as a result of this volume shrinkage, and this produces concave defects in the surface of the molded products, so that unsatisfactory products result.
A method has been proposed as a solution to this problem in which the mold is placed under pressure during molding, and the volume of the molding space (cavity) of the mold is reduced in accordance with the decrease in volume resulting from curing. However, in this molding method, an apparatus for applying pressure is required, and furthermore, when the walls of the mold are curved, the pressure does not act uniformly over the entire surface of the mold, and it is impossible to completely prevent concavities.
Accordingly, it is an object of the present invention to avoid the generation of concavities (voids) in the surface of molded products when heat-curable resin is molded using the casting method.
The present invention provides a method in which a central core member is placed in advance within a mold, heat-curable resin liquid is injected around the central core member within the mold, and when the mold is heated and curing of the resin is conducted, and an increase in volume is generated in the central core member which is in correspondence with the reduction in volume of the heat-curable resin as a result of curing. The present invention also provides a manufacturing method for heat-curable resin molded products in which three necessary factors are controlled: the selection of the central core member, the adjustment of the curing rate of the heat-curable resin, and the setting of the temperature of the mold.
Accordingly, in accordance with the manufacturing method for heat-curable resin molded products of the present invention, even when a heat-curable resin having a large amount curing shrinkage, such as unsaturated polyester resin or the like, is employed, it is possible to produce molded products having a satisfactory finish without concavities in the surfaces thereof. Furthermore, in comparison with the conventional manufacturing method in which pressure was applied to the mold, it is possible to prevent the generation of concavities even when the shape of the molded product is complex, having curved surfaces or the like. It is not necessary to apply pressure to the mold, so that it is possible to simplify the manufacturing installation, and it is also possible to extend the lifespan of the mold.